The embodiments herein relate to bearing assemblies and, more particularly, to a bearing sleeve assembly that may be used within a rotor system; namely within a rotary-wing aircraft.
Rotary wing aircraft include rotor systems and rotor blade assemblies to generate lift and allow for controlled operation of the air vehicle. During vehicle operation, the rotor blades are influenced by aerodynamic and inertial forces. Accordingly, each blade will experience elastic deformation as well as rigid body motion as a consequence of the forces acting upon it, referred to herein as blade dynamics. As a result of blade dynamics, rotor systems may be susceptible to forms of aero-elastic and aero-mechanical instabilities. In the pursuit of increased vehicle performance, new compound rotorcraft designs incorporate a coaxial rotor configuration with rigid rotor blades. A byproduct of such a configuration is that no appreciable relative motion occurs between the blade and the hub assembly, which precludes the ability to integrate a damping mechanism. Thus, aero-elastic stability is predominantly dictated by the combined elastic stiffness of the main rotor blade and hub assembly.
To address dynamic stability issues, it is desirable for significant separation to exist in the blade's natural frequencies (namely the first flatwise and edgewise modes). The primary means of ensuring frequency separation in under-damped systems is through the tailoring of stiffness in the degrees of freedom of concern. In the context of a rotor system, one area that has a significant effect on the edgewise and flatwise stiffness values is the hub assembly. Helicopters utilize bearings, which are contained within the hub assembly, to accommodate pitch changes of rotor blades. However, in a rigid rotor design the blade's flapping and lagging hinges are removed and made rigid. Thus, the blade bending moments imposed on the hub are increased in comparison to an articulated rotor configuration. The increased loading present in rigid rotor systems shifts the design towards the need to incorporate metallic pitch-bearing designs so as not to accommodate large radial load capacity as well as to ensure low impedance in accommodating changes in blade pitch. Rotary bearings of metallic construction incorporate cylindrical raceways that have an isotropic radial stiffness gradient through the element due to the symmetry of the design. Therefore, the isotropic radial stiffness properties of the bearing race results in equal stiffness values in the flatwise and edgewise directions. A challenge exists in the design of such a rotor system to accommodate the desirable traits of a rigid rotor system, while accommodating tailored stiffness values in the edgewise and flatwise orientations to avoid aero-elastic instability.